This invention relates to a multichannel remote temperature sensing data system comprising a reference temperature assembly for a number of remote temperature sensors.
Thermocouples are commonly used as temperature sensors in combination with datalogging equipment in applications where a number of temperatures at a number of remote locations are to be measured and recorded by a central datalogger. An example of a multichannel remote temperature sensing data logger of this type is described in U.S. Pat. No. 4,147,061, which is hereby incorporated in its entirety in the present specification.
A thermocouple, once calibrated, can measure accurately the temperature difference T.sub.D between its two junctions, especially where that temperature difference is small. In practice, however, an absolute temperature measurement of the hot junction of the thermocouple at temperature T.sub.H, is generally required. To this end, where a thermocouple terminates at a logger, its cold junction will be held at a reference temperature T.sub.C at the logger. T.sub.H is then known, being the sum of T.sub.C and T.sub.D.
Any uncertainty in the measurement of T.sub.C will thus influence the accuracy of measurement of T.sub.H . To obtain T.sub.C accurately in the highest performance dataloggers, platinum resistance thermometers (PRTs) are used to monitor the cold junction temperatures of each remote temperature sensing thermocouple. In a logging application, solid state electrical temperature sensors are clearly advantageous, and PRTs are used despite their high cost in preference to less costly thermocouples since they offer better accuracy. The best accuracy for thermocouples, to BS4937: part 20: 1983, is &lt;/-0.5+/-0.004T .degree.C., where T is the temperature in degrees Celsius. PRTs by contrast offer +/-0.1.degree. C. or better.
Thermocouples cannot therefore be used to monitor the absolute cold junction temperatures of sensing thermocouples if accuracy of 0.5.degree. C. or better is required. In practice therefore, for the highest performance, each sensing thermocouple terminated in a datalogger has its cold junction temperature monitored continuously by a PRT. Since a logger may have 50 or more sensing thermocouples, this is very costly.